The liquid crystal projector is mainly constituted of a light source, color separating optical elements, such as a dichroic mirror, liquid crystal elements, a color recombination prism, a projection lens. White light from the light source is separated through the color separation optical elements into light bundles of three primary colors. The three color light bundles are directed to the three liquid crystal elements respectively. The liquid crystal elements modulate the three color light bundles and output the modulated light bundles as optical images of three primary colors to the recombination prism, where the three color optical images are combined into a full-color optical image. The combined optical image is projected through the projection lens onto a screen, so the image is displayed on the screen. There are two types of liquid crystal projector: one uses reflective liquid crystal elements, and the other uses transmissive liquid crystal elements. In either type, a full-color image is displayed on the basis of three color optical images that are produced by the optical modulation at the liquid crystal elements.
As mentioned above, the projection lens is for projecting the combined optical image onto the screen. Since the fineness of images to be projected on the screen is getting higher these days, the projection lens is required to have a correspondingly high resolving power. In addition, for a rear-projection type projector that projects images from behind the screen, the projection lens is expected to be a wide-angle lens as having a shorter projection distance. The wide-angle projection lens is desirable also for the sake of minimizing the whole size of the projector.
However, because the projector using the reflective liquid crystal elements needs a polarizing beam splitter prism in addition to the color recombination prism, the distances between the projection lens and the liquid crystal elements are inevitably large. This is disadvantageous for the sake of widening the angle of the projection lens.
As a solution to this problem, it has been suggested mounting a front lens group and a rear lens group of the projection lens separately from each other in a projector. Because it is difficult to adjust the positions of the front and the rear lens groups to each other, and also because it is hard to increase the number of lens elements of the rear lens group, the front and the rear lens groups are conventionally held in their individual holders, and these lens holders are mounted on a prism base plate that holds the recombination prism.
In order to achieve adequate quality of the projected images, the positions of the front and rear lens holders on the prism base plate must be accurately adjusted not only in an axial direction but also in a vertical direction to the base plate. For this reason, the assembling process of the projection lens has been difficult.